What are the common causes of mild respiratory alkalosis in an adult?

Mild Respiratory Alkalosis: Common Causes in Adults

Mild respiratory alkalosis occurs when alveolar ventilation exceeds metabolic CO₂ production, resulting in decreased PaCO₂, elevated pH, and compensatory decreases in bicarbonate. 1

Primary Mechanisms

Respiratory alkalosis develops when hyperventilation eliminates more carbon dioxide than tissues produce, causing PaCO₂ to fall below normal (typically <35 mmHg) and blood pH to rise above 7.45. 2, 1 The condition triggers compensatory mechanisms including cellular uptake of bicarbonate and buffering by intracellular phosphates and proteins in acute cases, while chronic respiratory alkalosis leads to sustained renal decreases in bicarbonate reabsorption. 1

Common Etiologies

Psychogenic and Anxiety-Related Causes

• Hyperventilation syndrome and anxiety disorders are among the most frequent causes of mild respiratory alkalosis in the emergency department and outpatient settings. 3, 2
• Patients with psychogenic disorders (anxiety reactions, panic disorders, obsessional behavior) commonly present with exertional dyspnea, chest pain, and light-headedness due to unrecognized hyperventilation. 3
• Cardiopulmonary exercise testing in these patients reveals impressive hyperventilation evidenced by abnormal increases in minute ventilation, elevated Ve/VCO₂ ratio, increased respiratory frequency, and marked respiratory alkalosis with decreased PetCO₂ and PaCO₂. 3
• An abrupt "turned on" onset of regular, rapid, shallow breathing disproportionate to metabolic stress is characteristic, rather than the gradual increase seen during normal progressive exercise. 3
• Chronic respiratory alkalosis with a downregulated PaCO₂ set point may be observed at rest before exercise in these patients. 3

Pulmonary Causes

• Hypoxemia from any pulmonary disorder stimulates hyperventilation as a compensatory mechanism, producing respiratory alkalosis. 4
• Early or mild lung disease may present with isolated respiratory alkalosis before hypoxemia becomes severe enough to cause respiratory acidosis. 4

Cardiovascular Causes

• Respiratory alkalosis produces multiple cardiac effects including tachycardia, ventricular and atrial arrhythmias, and both ischemic and nonischemic chest pain. 4
• Hyperventilation during exercise has been associated with ECG changes resembling ischemia in subjects with normal coronary arteries. 3

Metabolic and Systemic Causes

• Sepsis and fever increase CO₂ production and stimulate hyperventilation, commonly resulting in respiratory alkalosis. 4
• Pain and increased work of breathing can drive hyperventilation and subsequent respiratory alkalosis. 4
• Pregnancy physiologically produces mild respiratory alkalosis due to progesterone-mediated increases in minute ventilation. 4

Iatrogenic Causes

• Mechanical ventilation with excessive minute ventilation settings directly causes respiratory alkalosis. 4
• Rebreathing from improperly configured breathing circuits or increased external dead space in ventilated patients can paradoxically contribute to alkalosis if compensatory hyperventilation occurs. 3

Diagnostic Approach

Arterial blood gas analysis is essential to confirm respiratory alkalosis, showing decreased PaCO₂ (<35 mmHg), elevated pH (>7.45), and compensatory decreased bicarbonate. 5, 1 The distinction between acute and chronic respiratory alkalosis is critical: acute cases show minimal bicarbonate compensation (typically >22 mmol/L), while chronic cases demonstrate more pronounced renal compensation with bicarbonate often falling to 18-22 mmol/L as the kidneys decrease bicarbonate reabsorption over 3-5 days. 1

Key Distinguishing Features

• Simultaneous arterial PCO₂ sampling is recommended when evaluating increased Ve/VCO₂, as this ratio may reflect either inappropriate hyperventilation or excessive dead space. 3
• In hyperventilation syndrome, irregular breathing patterns punctuated by breath holding (noted by changes in PetCO₂) and sighing are characteristic of hysteria and malingering. 3
• A complete, careful history and review of systems are essential for accurate interpretation, as hyperventilation syndrome is a diagnosis of exclusion. 3, 2

Clinical Significance and Metabolic Effects

Respiratory alkalosis produces multiple metabolic abnormalities affecting virtually every organ system. 4 Acute respiratory alkalosis causes clinically significant increases in plasma potassium (approximately +0.3 mmol/L) mediated by enhanced alpha-adrenergic activity and increased catecholamine concentrations. 6 The condition also affects phosphate, calcium handling, and can produce mild lactic acidosis. 4 In the gastrointestinal system, changes in perfusion, motility, and electrolyte handling occur. 4

Management Principles

Correction of respiratory alkalosis is best performed by treating the underlying etiology rather than attempting to directly manipulate ventilation. 4 For hyperventilation syndrome specifically, identification is important because appropriate treatment—including psychological counseling, physiotherapy and relaxation techniques, and potentially drug therapy—is usually successful. 3, 2 The therapeutic approach has several stages depending on severity: psychological counseling for mild cases, physiotherapy and relaxation for moderate cases, and drug therapy for severe or refractory cases. 2

Common Pitfalls

• Failing to recognize hyperventilation syndrome and pursuing extensive workup for organic disease delays appropriate treatment and increases patient anxiety. 3
• Overlooking the psychological component and focusing solely on respiratory mechanics misses the primary driver in psychogenic cases. 2
• In mechanically ventilated patients, failure to adjust ventilator settings appropriately perpetuates iatrogenic respiratory alkalosis. 4